1. Field of the Invention
The invention relates to a composite multilayer material comprising a metallic backing layer, a porous intermediate layer and an overlay applied thereto of PTFE or PTFE combined with other fluorothermoplastics, whose melting point is over 260xc2x0 C. and into which fillers are incorporated.
2. Description of Related Art
Although materials of this type may also be used without difficulty where no lubrication is present, the composite multilayer material according to the invention is particularly suitable for bearings using oil for which a very low coefficient of friction with simultaneous high wear and cavitation resistance is of great importance, such as in shock-absorbers for example, which are used in the automobile industry, or in gear pumps or hydraulic motors. Owing to the coefficients of friction required, such materials have, on the basis of current knowledge, to consist to a great extent of PTFE, since no other known plastics fulfill these requirements, even if suitable in their other characteristics.
Since PTFE is too soft when used alone, and therefore exhibits a high coefficient of wear, materials have to be mixed with it which counteract cavitation and wear without substantially increasing the coefficient of friction.
The bearing materials used most frequently in the above-mentioned field of application comprise a steel backing, a porous bronze layer and a plastics material, of which 80 vol. % is PTFE and the rest is lead or molybdenum sulphide. The plastics material is pressed into the bronze framework and forms a thin closed layer thereover. However, under extreme conditions, such as arise, for example, in shock-absorbers owing to high loads, high sliding speeds and acceleration rates and high flow speeds in the bearing gap, materials exhibiting this structure have a tendency to produce erosion and cavitation phenomena and therefore have only a limited service life, particularly under high levels of stress.
Although it is possible to produce materials which exhibit virtually no wear and absolutely no susceptibility to cavitation under the above-described conditions if a different thermoplastic material such as PVDF or PEEK for example is used instead of PTFE, a marked increase in friction has to be tolerated in such cases where oil lubrication is used.
Many documents propose PTFE-based compositions, but thus far these have represented only relatively slight improvements in relation to the above-mentioned standard materials and in most cases exhibit an increased coefficient of friction, such as in the case of calcium fluoride, disclosed in EP 0 183 375, or polyimides, which are proposed in DE 42 27 909 C2.
In GB 2272029 A, increased cavitation resistance is put down to the particularly large specific surface area of the lead powder used. However, the use of lead or lead compounds is regarded increasingly critically by the users of sliding elements and therefore does not constitute a wholly satisfactory solution.
Although the possibility of using poly-(p-phenylene-terephthalamide) as a filler in self-lubricating PTFE-based bearing materials is often mentioned, it is used without exception in the form of fibres, such as in GB 2291879 A for example.
WO 95/02772 describes a special type of fibrillated fibre, which is incorporated into the PTFE matrix. The fibrous form causes problems with homogeneous incorporation into the PTFE matrix and necessitates corresponding special production devices. Moreover, such fibres exhibit the disadvantage that they contain inhalable particles and are suspected of being carcinogenic.
The fibres are incorporated into the matrix in the manner of irregular wickerwork and so reduce shattering and erosion of the otherwise soft PTFE matrix.
References to aramid fibres repeatedly occur, e.g. in WO 97/03299 or GB 2177099 A, as a neutral element in fibrous form which may be replaced by other fibres and is therefore clearly not essential to the achievement of any particular characteristic.
A fundamental reason for the suitability and use of such fibres is their exceptional mechanical characteristic valuesxe2x80x94very high tensile strength and modulus of elasticityxe2x80x94which find their basis in the particularly high degree of molecular orientation in the longitudinal direction of the fibres and the strong physical parallel linkage of the individual rigid molecular strands.
The object of the invention is to provide a composite multilayer material which exhibits relatively high wear and delamination resistance without modification of the production process and without impairment of the coefficient of friction in the presence of oil.
This object is achieved with a composite multilayer material in which the overlay contains at least one powder-form polyaramid and in which the content of PTFE or of PTFE together with the other fluorothermoplastics amounts to 55-90 vol. %, the content of further fillers amounts to 9.5-44.5 vol. % and the content of polyaramid amounts to 0.5-10 vol. %.